This invention relates to a preparation process of crystallographically pure ferrierite-structure (FER) synthetic zeolite with crystal size homogeneously dispersed without the use of structural agent, of its use as an acidic catalyst, particularly for the frame-structural isomerization of linear olefins such as butenes of pentenes, and as a hydrocracking and hydroisomerization catalyst of linear, or barely ramified, long-chained paraffins, notably those that result from the Fisher-Tropsch reaction from the synthesis gas (CO+H.sub.2).
It is known, in the field, that the materials called zeolites, may they be natural or synthetics, possess catalytic properties for the conversion of diverse types of hydrocarbons. Typically, these materials are porous crystalline alumino-sillcates whose crystalline structure is well defined, and contain a large number of very small canals or pores of a uniform size, adapted for the absorption of specific-size molecules. Ferrierite zeolite is among these natural alumino-silicate zeolites.
A great variety of ways to obtain synthetic zeolites have been previously described, in particular zeolite A (U.S. Pat. No. 2,882,243), zeolite Y (U.S. Pat. No. 3,130,007), and the so-called zeolite ZSM-5 (U.S. Pat. No. 3,702,886).
Likewise, U.S. Pat. No. 4,016,245 describes the synthesis of a alumino-silicate crystalline zeolite, named ZSM-5, that possess a crystallographic structure very similar to that of the ferrierite and that can be used as a catalyst for the conversion of hydrocarbon conversion. Nevertheless, this synthesis always involves an organic-nitrogen compound such as ethylenediamine or pyrrolidine, which behaves like a structuring agent, orienting the synthesis towards a predetermined structure. This zeolite has a orthohombic network made of two series of unidimensional canals that cross perpendicularly, one series being composed of canals of 10 "T atoms" (tetrahedric Si and AJ atoms) of slightly elliptic section (4.2.times.5.4 .ANG.), and the other of canals of 8 "T atoms" (3.5.times.4.8 .ANG.).
Other patents show a process of fabrication of this material which uses diverse structuring agents such as alkylpyridines (U.S. Pat. No. 4,251,499) or alkylpyperidines (U.S. Pat. No. 4,795,623) or other agents.
It is known, by the demand for Patent EP 0523 838 (Lyondell), that it is possible to use a process of skeletal isomerization of linear olefines and isoolefines, such as butene-1 or pentene-1, a catalyst of zeolite type having a structure and a dimension of pores adapted especially to reduce parasitic reactions and cokage. That document describes many types of zeolites, ferrierites among them, but does not mention the process of their manufacture.
A recent study by Wen Quing Xu et al., published in J. Phys. Chem. 1995 Vol. 99, 9443-9451, which compared the structures of different zeolites ZSM-5 of ferrierite type, synthesized respectively with a structuring agent (pyrrolidine) and without this agent, having a Si/Al ratio between 5.7 and 9.2, shows that the first category presents particles whose size displays a uniform distribution whereas the size of the crystallines in the second category is very heterogeneous, varying from a few .mu.m to 100 .mu.m. Furthermore, the zeolites of the first category present a higher crystallinity and contains less mesoporosites than the zeolites obtained without structuring agent. The result is an augmentation of the selectivity in skeletal isomerization of linear butene in isobutene for the zeolites synthesized with structuring agent.
This study further explains that the successful synthesis of ferrierite-type ZSM-35 zeolite, without using organic structuring agents, requires the presence of precursory gels that have a narrow Si/AI margin (roughly equal to 9) and agitating during the autoclave treatment.